add TSamFailedOnly command and change test output within the floating window

tests/test_samurai_commands_spec.lua

@@ -0,0 +1,15 @@
+describe("test-samurai commands", function()
+  before_each(function()
+    vim.g.loaded_test_samurai_plugin = nil
+    vim.cmd("runtime plugin/test-samurai.lua")
+  end)
+
+  it("registers TSamFailedOnly command", function()
+    assert.equals(2, vim.fn.exists(":TSamFailedOnly"))
+  end)
+
+  it("registers <leader>te keymap", function()
+    local map = vim.fn.maparg("<leader>te", "n")
+    assert.is_true(map ~= nil and map ~= "")
+  end)
+end)